Event Summary
     National Weather Service, Raleigh NC

November 18-19, 2000 Winter Storm

Event Overview -
  • Description - An area of wet snow fell across northern Georgia - upstate South Carolina - into central North Carolina on Sunday November 19, 2000. 1-3 inches of snow was the general rule for much of this area. The accumulation of snow was limited by relatively warm soil temperatures. Some of the snowfall melted as it fell. A total of nearly 0.70 inches of precipitation fell at the RDU airport. About 0.50 of this amount was in the form of snow. Snow accumulated to around 1.5 inches at the airport. An accumulation of snow at RDU this early in the winter season is climatologically unusual.

Total snowfall accumulation 11/18/00 through 11/19/00
Amounts are in inches and "T" represents trace amounts.



  • Pattern of Cyclogenesis - (Active Southerly Jet, west-east surface ridge, ill defined surface wave)
    Surface analysis indicated a weak west to east high pressure ridge extended across the southern plains into Kentucky and a poorly defined surface wave was near the Gulf Coast. Little or no horizontal thermal advection is associated with such weak meteorological features. In the absence of horizontal thermal advection, diabatic processes determine the p-types. The stretching of the surface ridge from west to east is indicative of an active upper level southerly jet. In summary, the analysis of surface data provided the following clues:

    • weak wave and ridge axis - lack of horizontal thermal advection so diabatic processes will determine the p-types.
    • west to east surface ridge axis - an active upper level southerly jet
    • measurable precipitation can occur farther north than you might think - i.e, near the centers of weak high cells along the ridge axis.

11/19/00 1200Z 300 mb wind and height (ETA analysis)




11/19/00 1200Z 850 mb temperature and height (ETA analysis)




11/19/00 1200Z Surface pressure and thickness (ETA analysis)






  • Soundings (Good potential for cooling) - Both real-time and projected soundings showed the advection of moisture to be well aloft with plenty of surface based dry air below. Cooling initially from evaporative cooling and later from the melting of snow could potentially produce a vertical temperature profile supporting snow should there be sufficient precipitation rates. In summary, the soundings provided the following clues:

    • advection of moisture well aloft and associated with the southerly jet
    • strong potential for cooling from evaporation and melting could produce a sounding supporting snow should sufficient precipitation rates occur.


Observed Greensboro (KGSO) Sounding valid 06Z 11/19/00




Observed Greensboro (KGSO) Sounding valid 12Z 11/19/00



  • Composite Partial Thickness (No icing) - Isopleth of 1000/850 and 850/700 partial thickness were nearly parallel with little overlapping. Such of pattern is indicative of gradual and/or nearly isothermal vertical temperature profiles. Such profiles do not support significant sleet or freezing rain. In summary, the patten of composite partial thicknesses provided the following clue.

    • Nearly parallel partial thickness isopleth are indicative of gradual or nearly isothermal vertical temperature profiles suggesting liquid and/or frozen precipitation. Icing in the form of sleet and/or freezing rain requires overlapping partial thickness isopleth indicating inversions from mid-level warm air overlapping surface based cooler air.


  • Partial Thickness Values (thermal interpretations) - 850-700 mb partial thickness values at GSO were generally expected to be in the 1545 - 1555 meter range while the 1000-850 values were expected to be from about 1295-1305. The 1545 - 1555 range is associated with a significant portion of the 850-700 layer above freezing or a relatively deep near freeing isothermal layer. The 1295 - 1305 range is indicative of small to significant portion of the 1000-850 layer above freezing. In summary, the partial thickness values provided the following clue.

    • The 1545 - 1555 range in the 850-700 mb layer prompts the forecaster be alert for the development of a relatively deep near freezing isothermal layer. With the occurrence of such a layer, then the 1295-1305 range in the 1000-850 mb layer means there will be a surface based above freezing portion in that layer.
    • The combination of a relatively deep near freezing isothermal layer with a surface based above freezing portion in the 1000-850 layer supports a rain/snow p-type. In the absence of horizontal thermal advection, the issue of whether snow reaches the ground is dependent upon the precipitation rate.
    ETA grid point forecast data vaild at Greensboro (KGSO)

    11/18/00 00Z Eta Forecast 1000 - 850 mb Thickness (m) 850 - 700 mb Thickness (m)
    24 Hour Fcst valid 11/19/00 00Z 1306 1552
    36 Hour Fcst valid 11/19/00 12Z 1303 1551
    48 Hour Fcst valid 11/20/00 00Z 1305 1548

    11/18/00 12Z Eta Forecast 1000 - 850 mb Thickness (m) 850 - 700 mb Thickness (m)
    12 Hour Fcst valid 11/19/00 00Z 1306 1551
    24 Hour Fcst valid 11/19/00 12Z 1304 1551
    36 Hour Fcst valid 11/20/00 00Z 1304 1545

    11/19/00 00Z Eta Forecast 1000 - 850 mb Thickness (m) 850 - 700 mb Thickness (m)
    00 Hour Fcst valid 11/19/00 00Z 1306 * 1549
    12 Hour Fcst valid 11/19/00 12Z 1303 1543
    24 Hour Fcst valid 11/20/00 00Z 1306 1544

    11/19/00 12Z Eta Forecast 1000 - 850 mb Thickness (m) 850 - 700 mb Thickness (m)
    No Fcst valid 11/19/00 00Z
    00 Hour Fcst valid 11/19/00 12Z 1306 * 1545
    12 Hour Fcst valid 11/20/00 00Z 1305 1539



    Observed data at Greensboro (KGSO)

    GSO Raob Time 1000 - 850 mb Thickness (m) 850 - 700 mb Thickness (m)
    11/18/00 12Z 1290
    1537
    11/19/00 00Z 1305
    *1547
    11/19/00 06Z 1310
    1550
    11/19/00 12Z 1305
    *1540
    11/20/00 00Z N/A
    N/A


    *Note that the Eta forecast 850 - 700 mb thickness at 11/19/00 00Z and 11/19/00 12Z are initialized too warm at Greensboro. The model initialized conditions are weighted and then blended with a model's most recent forecast. The most recent forecast is used as a first guess of initial conditions. The weighting of the actual observations and then the blending with the model's first guess is done through a model's objective analysis scheme.


  • TRENDS & AREA Techniques on BUFKIT (p-type tools) - Using the partial thicknesses from the eta model, the TRENDS technique showed the partial thicknesses generally in the "snowy nose" portion of the nomogram for GSP, CLT, GSO, and RDU. The appearance of the "snowy nose" triangle on the nomogram prompts the forecaster to expect a relatively deep near freezing isothermal layer supporting measurable snow with rain. The area technique forecast rain throughout the event except at GSO where a snow/rain mix was indicated. Since the eta model maintained a surface based above freezing portion in the 1000-850 layer, the AREA technique did not capture the snow event.

    In summary, TRENDS offered the following guidance:
    • Evaporative cooling and melting of snow will cool the atmosphere enough to support a measurable snow event that occurs from the changeover of rain to snow.

    Evidently, the low-level details in the Eta's forecast soundings were too warm, with the lowest 1500 feet remaining above freezing through the period. The loop below is from the ETA forecast for Raleigh (KRDU) from the 11/19/00 00Z ETA with each frame depicting even hours from 11/19 00Z through 11/19 18Z. The model depicts a cooling of the temperature (red line) as the lower atmosphere moistens during the period from 00Z to 06Z. The cooling becomes more substantial as precipitation begins after 06Z. Note that the freezing level drops from 5000 feet to about 1500 feet during this 12 hour period.

    ETA Forecast sounding for Raleigh (KRDU) from 11/19/00 00Z ETA valid 11/19 00Z - 11/19 18Z



    Note - The red line is the forecast ambient temperature, the green line is the dew point temperature and the blue line represents the wet bulb temperature. The light gray line is the omega field with negative values (to the left) representing upward vertical motion. The vertical axis is in thousands of feet.


    Meteogram of observed precipitation type at KRDU



    Each symbol represents the precipitation type observed at KRDU during the period from 11/19/00 12Z to 11/20/00 00Z.


    KRDU Metar's for the same period

    METAR KRDU 190151Z COR 27003KT 10SM BKN100 05/00 A3020 RMK AO2 SLP214 T00530000
    METAR KRDU 190351Z 24003KT 10SM BKN080 05/01 A3015 RMK AO2 SLP214 T00500006
    METAR KRDU 190551Z 00000KT 10SM BKN110 04/01 A3016 RMK AO2 SLP217 10050 20044 T00440006 53004
    METAR KRDU 190751Z 25004KT 10SM SCT070 04/00 A3016 RMK AO2 SLP218 T00440000 VISNO RWY 23R $
    METAR KRDU 190951Z 00000KT 9SM BKN085 04/01 A3017 RMK AO2 SLP220 T00440006 $
    METAR KRDU 191151Z 00000KT 9SM OVC065 04/01 A3019 RMK AO2 SLP228 10044 20039 T00390011 53005 $
    SPECI KRDU 191355Z 00000KT 10SM FEW049 OVC060 04/01 A3018 RMK AO2 PLE55 P0000 $
    METAR KRDU 191551Z COR 05005KT 4SM -SNRA BR OVC011 02/01 A3023 RMK AO2 SLP239 P0002 T00170011 $
    METAR KRDU 191751Z 00000KT 1/2SM SN FG OVC004 01/01 A3018 RMK AO2 RAE24 SLP224 60007 P0003 T00060006 10122 20006 58015 $



  • Forecast Soundings - The red line is the forecast ambient temperature, the green line is the dew point temperature and the blue line represents the wet bulb temperature. The light gray line is the omega field with negative values (to the left) representing upward vertical motion. The vertical axis is in thousands of feet.

18 Hour ETA Forecast for Greensboro from 11/19/00 00Z ETA valid 11/19/00 18Z



12 Hour ETA Forecast for Greensboro from 11/19/00 06Z ETA valid 11/19/00 18Z



06 Hour ETA Forecast for Greensboro from 11/19/00 12Z ETA valid 11/19/00 18Z

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